1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive composition, a polarizing plate, and a liquid crystal display.
2. Discussion of Related Art
A polarizing plate is a functional optical member included in a liquid crystal display (LCD). The polarizing plate has a multi-layered structure including a polarizer having iodine compounds or dichromatic polarizing materials arranged in a predetermined direction, and a protective film, such as triacetylcellulose (TAC), that is formed on both sides of the polarizer and functions as protecting the polarizer. Further, the polarizing plate may include an additional film such as a retardation plate, a wide viewing angle compensation plate, or a brightness enhancement film in view of functional improvement.
Each film constituting the multi-layered polarizing plate is made of a material having a different molecular structure and composition, and thus exhibits different physical properties. Therefore, the polarizing plate has inferior dimensional stability especially under high temperature and/or high humidity condition, since the film that has an unidirectional molecular arrangement and constitutes the polarizing plate shows a difference shrinkage or expansion behavior. Therefore, when the polarizing plate is fixed by a pressure-sensitive adhesive, birefringence is induced and light leakage is arisen with stress is concentrating on the TAC layer, and the like, under high temperature and/or high humidity.
To solve these problems, techniques for securing stress relaxation properties of a pressure sensitive adhesive by designing it so as to have high creep and be easily deformed, with respect to external stress have been disclosed (e.g. Korean Patent Application Publication No. 1998-079266, Japanese Patent Application Publication Nos. 2002-047468 and 2003-049141, and the like.).
However, the pressure-sensitive adhesive disclosed in the above techniques shows inferior cuttability, so that depression or sticking-out of the pressure-sensitive adhesive easily occurs when manufacturing the polarizing plate, and thus productivity and yield are remarkably reduced.
As another method of preventing the light leakage, methods, in which the pressure-sensitive adhesive is designed to be very hard is known. When the pressure-sensitive adhesive is designed to have a rigid physical property, the shrinkage and expansion of the polarizing plate under high temperature and/or high humidity are inhibited to a maximum extent, so that it is possible to minimize generation of stress, concentrate the stress on the outermost edge, and achieve relatively excellent optical properties.
For example, Japanese Patent Application Publication No. 2007-197659 discloses a technique of manufacturing a pressure-sensitive adhesive composition by formulating an acrylic resin having carboxyl group, multifunctional acrylate, an isocyanate hardener and a photo-initiator so as to prepare a pressure sensitive adhesive composition, and curing it by UV irradiation. Japanese Patent Application Publication No. 2007-212995 discloses a technique of manufacturing a pressure-sensitive adhesive composition by mixing a copolymer containing a hydroxy group with a copolymer containing a carboxyl group at a predetermined ratio, and adding multifunctional acrylate, a multifunctional isocyanate hardener and a photo-initiator so as to prepare a pressure sensitive adhesive composition, and curing it by UV irradiation.
However, the pressure-sensitive adhesives disclosed in the above documents, have problems that its initial adhesion strength is largely reduced according to its storage modulus, G′, and thus its durability is greatly reduced.